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    <meta charset="UTF-8">
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    <title>Document</title>
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<body>
    <script>
        let str = 'sjdlfj23423ls中4434国fsdk'
        // 正向断言
        let reg = /\d+(?=国)/
        let result = reg.exec(str)
        console.log(result)

        // 反向断言
        reg = /(?<=中)\d+/
        result = reg.exec(str)
        console.log(result)
    </script>
</body>
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